What is Medullary Index and Why Should You Care?
The Medullary Index (MI) is the ratio of the diameter of the medulla (the innermost part of the hair shaft) to the diameter of the hair itself. Whether you're a forensic analyst trying to determine the origin of hair samples or a scientist studying the structural properties of different hair types, the Medullary Index offers valuable insights.
Hair can be essential evidence in criminal investigations or can reveal interesting biological details about different species. Human hairs often have different medullary indices than animal hairs, giving crucial clues in forensic cases.
How to Calculate Medullary Index
The formula is straightforward:
[\text{MI} = \frac{\text{Diameter of the medulla}}{\text{Diameter of the hair}}]
Where:
- Medullary Index (MI) is the ratio of the medulla to hair diameter
- Diameter of the medulla is the width of the innermost part of the hair
- Diameter of the hair is the total width of the hair strand
Steps to Calculate
- Determine the Diameter of the Medulla: Use a microscope to find the diameter of the medulla in your hair sample
- Determine the Diameter of the Hair: Measure the total diameter of the hair strand
- Insert the Values into the Formula: Divide the medulla diameter by the hair diameter
- Calculate the Medullary Index: The result is your Medullary Index
Calculation Example
Suppose we're analyzing a hair sample and we measure:
- Diameter of the medulla = 2 mm
- Diameter of the hair = 5 mm
Using our formula:
[\text{MI} = \frac{2 \text{ mm}}{5 \text{ mm}} = 0.4]
The Medullary Index is 0.4, indicating that the medulla is 40% of the total diameter of the hair in this sample.
Why This Matters
A Medullary Index lower than 0.3 is typical for human hair, while a higher value is often observed in animal hair. Knowing this can help forensic experts rule out or confirm human involvement based on found hair samples.